This invention relates to the formation of porous articles. More particularly, it relates to the formation of porous ceramic articles and porous metal articles.
In the production of certain articles for use in many applications, such as refractory, kiln furniture, filtration, fuel cell, bone implant, catalyst substrates, catalysts, particulate traps, filters, diffusion layers, electrical conductors, heat exchange components, wicks for heat pipes, wicks for burners, radiant burner surfaces, diffusion layers for introducing fuel and/or water into an air stream, it is sometimes desirable to reduce the overall density of the fabricated article by introducing porosity into the article during or after fabrication. The strategy employed for reducing the mass of the article after fabrication usually involves removal of material from the article by means of grinding, drilling, routing or other mechanical methods to physically remove material from selected locations. This usually takes the form of drilling holes, routing channels, etc. Reducing the mass of the material (per unit volume of space occupied by the fabricated article) during fabrication involves using a process which introduces porosity into the material. This can be accomplished by various methods described in the literature.
Some of the basic patents assigned to Selee Corporation, assignee of the present invention, disclose a method to produce a ceramic foam article with a high volume percent interconnected porosity by impregnating a reticulated polyurethane foam with a ceramic slurry, made primarily from ceramic powder, a binder and water, and heating the impregnated polyurethane foam to bum of the polymer and sinter the ceramic. This method can be used to produce various pore sizes and densities. The reported strengths for various ceramic materials fabricated in this manner lie in the 100-700 psi range.
Another method to produce low density ceramic kiln furniture is taught in U.S. Pat. No. 4,812,424, whereby a porous aluminosilicate refractory aggregate is fired. The aluminum metal, alkali silicate and alkali aluminate chemical reaction producing a large volume of small gas bubbles is combined with a sodium silicate-sodium aluminate hydrogel setting reaction which traps the hydrogen gas bubbles in the ceramic. The strengths of this material are approximately in the 500-1000 psi range.
U.S. Pat. Nos. 4,814,300, 4,846,906, 4,871,495, 4,878,947, 4,923,487, 4,963,515 and 4,976,760 are extensions of this basic technology to include membranes and are used in specific markets, such as diesel particulate traps and diesel filters.
European Patent Specification Publication No. EP 0 598 783 B1 discloses a method of preparing porous refractory articles by forming a dispersion comprising particles in a liquid carrier, introducing gas into the dispersion and removing the liquid carrier to provide a solid article having pores derived from the bubbles.
U.S. Pat. No. 4,889,670 discloses a method to produce porous ceramic parts by combining a mixture of 60-90 weight percent of a particulate ceramic with 10-40weight percent of a latex polymer, whereby the mixture is frothed by mechanical means, shaped, set and sintered to produce the porous article.
It is also well known that porosity can be introduced into a ceramic article by incorporating various types of organic particles into the ceramic body. Upon firing, these particles are oxidized and leave behind voids in the material.
Porous metal foam articles have been developed by Astro Met, Inc., of Cincinnati, Ohio, and are disclosed in U.S. Pat. 5,937,641, issued to Graham et al. Porous metal foam articles are made using a process which is similar to the process used by SELEE Corporation in manufacturing its ceramic foam articles, however, the ceramic powder is replaced with metal powder as a starting material. The Graham patent discloses that the porous metal foam may be used as a catalytic core element or a catalytic element for a catalytic converter.
It is, therefore, one object of this invention to provide improved porous articles which are stronger, more thermally shock resistant, possesses uniformly dispersed and highly controlled pore sizes, and which can be made more quickly and economically than presently available materials, such as foam materials.
It is another object of this invention to provide an improved method to produce ceramic articles so that the size and size range of the pores, and the size and size range of the interconnections between the pores, can be more closely controlled than with currently available techniques.
It is still another object of this invention to provide an improved method to produce porous articles so that the volume percent of the porosity and the distribution of pores throughout the articles can be closely controlled.
It is further another object of this invention to provide an improved method to produce porous articles so that the porosity extends to and through the surface of the articles rather than forming a solid skin on the surface.
It is yet another object to provide a porous ceramic and metal articles in accordance with the above methods.
It is also another object to provide a method for producing a coated porous ceramic article which will retain its coating through a large number of thermal cycles.
In accordance with one form of this invention, there is provided a method for forming a porous article. A mixture of ceramic or metal particles and pliable organic spheres is prepared in a liquid. Preferably, a suspension of the particles and pliable organic spheres is formed. Preferably, the spheres are hollow and are made of a polymer, such as acrylic. The mixture is formed into a shaped article. The shaped article is dried. The shaped article is then fired so that the particles are bonded such as by sintering, and the pliable organic spheres are eliminated, resulting in voids in the shaped article. If the article is ceramic, the firing may take place in an oxygen rich atmosphere so that the organic spheres are eliminated primarily by oxidation. However, if the article is metal, the firing should take place in a very low oxygen environment to avoid oxidizing the metal and thus the organic spheres are substantially volatilized. That is, the organic compound disassociates and decomposes into gaseous species in order to avoid oxidation of the metal. To make it easier to volatilize the spheres, it is preferred that the spheres are low density, e.g., hollow.
In accordance with another form of this invention, there is provided another method for producing porous ceramic articles. A suspension of ceramic or metal particles and pliable organic hollow spheres are formed such that the particles and pliable hollow polymer spheres are simultaneously suspended in a liquid, preferably including water. A shaped article is formed, after a sufficient amount of water is added, either slip casting, pressing, extrusion, or injection molding. The shaped article is dried to remove the water. The shaped article is then fired to allow bonding of the particles such as by sintering, and to eliminate the pliable organic hollow spheres, resulting in uniformly distributed voids in the shaped article.
A range of porosities of up to 95% void volume may be achieved using these methods. The size of the voids may be preselected by selecting the appropriate size polymer spheres. The amount of porosity is easily controlled by the number of polymer spheres which are added. The size range of the pores can be closely controlled by controlling the size range of the polymer spheres which are used The distribution of the pores in the article is highly uniform due to the fact that the polymer spheres and the particles are preferably simultaneously suspended by the addition of the appropriate suspending agent.
If the article is ceramic, it may be coated, for example with another ceramic composition. It has been found that the coating will stay bonded to the article through a large number of thermal cycles. Similar coatings on other substrates do not adhere as well.
In accordance with another form of this invention, there is provided a porous ceramic or metal article having a plurality of substantially spherical shaped voids. The voids are substantially uniformly dispersed throughout the article. The voids are interconnected with one another. For a ceramic article having a theoretical density in the range from 5% to 30%, the strength of the article is in the range from 700 psi to 4500 psi.
In another form of this invention, there is provided a porous ceramic or metal article having a plurality of substantially spherical shaped voids therein. The voids are substantially uniformly dispersed throughout the article. The voids are interconnected with one another. A substantial number of the voids intersect with at least one adjacent void. A window is formed by the intersection. The window is substantially in the shape of a circle. The average diameter of the circular windows is in the range from approximately 11 microns to 22 microns.
In accordance with another form of this invention, there is provided a porous ceramic article having a plurality of substantially spherical shaped voids therein. The voids are substantially uniformly dispersed throughout the article. The voids are interconnected with one another. At least a portion of the article is coated. The coating may comprise a ceramic composition, such as zirconia. The coating will remain adhered to the article for at least ten thermal cycles. The preferred thermal cycle is from approximately room temperature to approximately 2200xc2x0 F. and back to approximately room temperature in approximately 2 hours.